1. Field of the Invention
The present invention is directed to a current sensor operating according to the compensation theorem, particularly for measuring direct and alternating currents.
2. Description of the Prior Art
In a current sensor which operating according to the compensation theorem, the magnetic field generated in a magnetic core by a primary winding through which the current to be measured flows is compensated by the compensation current in a secondary winding. At least one sensor influenced by the magnetic field measures deviations from zero flux for controlling the compensation current, and supplies this measured value via an evaluation circuit to an amplifier arrangement for generating the compensation current. The secondary winding, in series with a terminating resistor, is connected to the output of the amplifier arrangement, so that a voltage proportional to the current i1 to be measured is present across the terminating resistor.
A current sensor of this type operating according to the compensation theorem is disclosed, for example, in European Application 0 356 248. The schematic circuit of this known current sensor is shown in FIG. 1 herein. A current to be measured flows through the primary winding 1 of a current transformer that has a magnetic core 2 as well as a sensor 3 that measures the magnetic flux in the magnetic core 2.
In the arrangement disclosed in European Application 0 356 248, the sensor 3 is composed of a transformer driven into saturation and having a rectangular magnetization characteristic. In principle, any desired sensor for a magnetic field can be employed, for example, a Hall probe, an amorphous strip with a winding, etc. The output voltage of the sensor 3 is evaluated in a following evaluation circuit 4 whose output is connected to the input of an amplifier arrangement 5. The output of the amplifier arrangement 5 is connected to ground via the secondary winding 6 of the current transformer and via a terminating resistor 7.
The functioning of this arrangement is as follows: Via the primary winding 1, the current to be measured generates a flux in the magnetic core 2 that is acquired by the sensor 3. The evaluation circuit 4 that follows the sensor 3 supplies a signal that is dependent on the size and direction of the magnetic field in the magnetic core 2 to the amplifier arrangement 5. The amplifier arrangement 5 drives a compensation current through the secondary winding 6. The compensation current is directed such that its magnetic field compensates the magnetic flux in the magnetic core 2. The current in the secondary winding 6 is modified by the sensor 3 until the magnetic field in the magnetic core 2 becomes zero. The current in the secondary winding 6 is thus a criterion for the momentary value of the current to be measured in the primary winding 1, so that direct as well as alternating currents can be measured. This current also flows across a terminating resistor 7 across which the output voltage U.sub.a of the current sensor drops, this voltage drop corresponding in size and phase relation to the current to be measured in the primary winding 1.
Since this output voltage U.sub.a --dependent on the direction of the current to be measured--must assume positive as well as negative values, a current will flow--dependent on the direction of the current--either from the positive pole .sym. of the supply voltage of the amplifier arrangement via the secondary winding 6 and the terminating resistor 7 or--given an opposite direction of the current in the primary winding 1--a current will be supplied from the minus pole e of the amplifier arrangement 5.
Given this arrangement, the maximum output voltage U.sub.a is dependent on the size of the supply voltage of the amplifier arrangement 5 as well as on the value of resistance of the terminating resistor 7 and on the value of resistance of the ohmic internal resistance of the secondary winding 6. Particularly when relatively high currents are to be measured, the secondary winding 6 must have many turns, so that the internal resistance thereof necessarily increases. Because the supply voltage of the amplifier arrangement 5 cannot be arbitrarily raised, it is necessary for measuring very high currents as occur, for example, in circuits of electric cars, to either reduce the internal resistance of the secondary winding 6 by using a thicker wire, or the terminating resistor 7 must be made smaller. Using a thicker wire has the disadvantage that the current transformer composed of the primary winding I and the secondary winding 6 becomes relatively large. Using a smaller terminating resistor 7, however, has the disadvantage that the dissipated power that is required for generating the compensation current in the secondary winding 6 is increased.